1. Field of the Invention
The present invention relates to an active inductor, i.e., an electronic circuit simulating the behaviour of an inductance by using only suitably biased active components, capacitors and resistors.
2. Description of the Related Art
An active inductor is, as such, an inherently noisy circuit, which therefore would not be suitable to process signals with very low power, since the signal/noise ratio would be excessively degraded by the processing itself.
A first type of prior art active inductors consists of impedance simulators using operational amplifiers (OPAMPs), transconductance amplifiers (OTAs), or current conveyors (CCs) as active components. This type of active inductors has maximum operating frequencies of a few MHz, and it is especially used in low and very low frequency applications.
In order to obviate this limitation in frequency, a second type of active inductors of a known type has been developed, in which such active inductors are mostly implemented in CMOS technology. This type of active inductors is characterized by higher operating frequencies (up to some GHz), but it is often characterized by a high parasitic series resistance that, in order to reduce losses, is controlled by circuits referred to as NICs (Negative Impedance Circuits). Such circuits implement methods that make the inductance variable by changing the gain of the active elements composing the inductor by varying the biasing conditions thereof. This class of active inductors is characterized by low losses, by virtue of the use of NIC circuits; however, the methods that are used to implement the inductance variability cause a high distortion already at very low powers. Therefore, such active inductors cannot be used in processing signals having the powers generally used in radio-frequency transmissions, thus they are suitable only to process signals with a power in a very narrow range.
In order to obviate this drawback, a third type of active inductors has been developed, which is composed of active inductors having an inductance with fixed value, which use phase compensation networks in order to reduce series resistance. This type of active inductors, combined with the use of transistors (for example, of the BJT type) as active elements, is potentially capable to process high signal powers with low distortion. However, as it is known, in a tunable filter consisting of inductances and capacitances, the variation in the center frequency, while keeping a given Q factor (Quality factor), requires varying at the same time both the inductances and the capacities composing the filter. This is not possible with the inductors of such third class, since they are devices characterized by a preset inductance.
Furthermore, it is noticed that each of the types of active inductors of the described prior art has also a rather high noise figure.